<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Responsive Size Demo</title>
    <style>
        .container {
            width: 50vw;
            height: 60vh;
            background-color: aquamarine;
        }
        .child {
            width: 30%;
            height: 100px;
            background-color: burlywood;
        }
    </style>
</head>
<body>
    <div class="container">
        <div class="child"></div>
    </div>
</body>
</html>